The present invention relates to laminated paper polyolefin paper composites used in high voltage cables. These composites are formed by bonding together fiber paper layers of paper grade cellulose and a polyolefin layer. The invention relates more specifically to composites which have increased discharge resistance and which swell less than conventional composites when contacted with insulating oils.
Polyolefin laminate paper has good dielectric characteristic and high dielectric strength. Therefore, it is often used as an insulating material for high voltage or extra high voltage cables. However, various difficulties are present in conventional polyolefin laminates which prevent their use as replacements for more common paper insulation for transmission cable applications. One of these difficulties is the occurrence of partial discharges when there are flaws or air spaces in the laminates. Another difficulty is the swelling of the polyolefin layer which occurs when it is contacted with insulating oils. This swelling causes the tape not to slide and therefore wrinkle; it also leads to a reduction in dielectric strength of the polypropylene.
Various methods have been used to attempt to provide a superior laminated insulation construction. A first method is an "annealing method" where a cable, after taping, is subjected to a heat treatment carried out in a vacuum at a temperature lower than the melting point of polyolefin for a predetermined period of time to increase the degree of crystallinity of the polyolefin film, and thereby decrease swelling in the insulating oil. A second method is a "humidity adjusting method" where the paper layers are caused to absorb moisture to expand in advance; the decrease in thickness of the paper layers caused by drying after the taping operation is utilized to counter the increase in thickness of the polyolefin film layer which is caused by the swelling. A third method is an "after-embossing method" in which a paper polyolefin laminate is embossed so that irregularities are mechanically formed thereon. Deformation of these irregularities absorbs the increase in thickness of the polyolefin layer when it is swelled in the insulating oil. A fourth method is a "pre-embossing method" in which the paper layers are embossed prior to lamination.
It is an object of the present invention to provide laminate paper polyolefin paper composites which are discharge-resistant.
It is a further object of the present invention to provide laminate paper polyolefin paper composites which exhibit reduced swelling on contact with insulating oils.
Methods for making the above-referenced composites are also provided as yet a further object of this invention.
It is yet a further object of the present invention to provide high voltage cables with improved discharge-resistance and swelling characteristics. This in turn will lead to longer life and greater reliability.